 :ext - An overriding file extension (as keyword) to force a particular
        file type to be opened instead of looking at the files extension."
 
-  (fn [file {:keys [format]}]
-    (if (#{File String} (class file))
-      (or format (extension file))
-      (class file))))
+  format-or-type)
 
 (defmethod open-dataset* Dataset [dataset opts]
   dataset)

  (fn [multidatasetable {:keys [format] :as opts}]
    (when (:sheet opts)
      (throw (IllegalArgumentException. "open-datasets cannot open a single sheet.  Use open-dataset* to do this.")))
    (or format
        (extension multidatasetable))))

(defn open-datasets
  "Opens a lazy sequence of datasets from a something that returns multiple

        (.substring 1)
        keyword)))

(defn format-or-type [file {:keys [format]}]
  (if (#{File String} (class file))
    (or format (extension file))
    (class file)))

(defmulti open-dataset*
  "Opens a dataset from a datasetable thing i.e. a filename or an existing Dataset.
The multi-method dispatches based upon a :format option. If this isn't provided then
